West Africa is vast, an area larger than the continental United States. It contains several broad bands of different ecosystems that stretch, roughly in parallel, across the region east to west and encompass extremes in temperature and rainfall. West Africa is bordered on the north by the Sahara Desert and in the south by the heavily urbanized southern coast interspersed with mangroves, tidal estuaries, and patches of dense, moist rainforests. Off the west and south coasts the Canary and Guinea Currents are critical elements of the marine ecosystems of the seas washing the coastline. The biological diversity is substantial with the highest gradients of richness in the Upper Guinean Forest of the south-central portion of West Africa and in the Canary Current off the western coast. The Sahel, the 'margin' or 'edge' in Arabic, is the transition grassland, shrub forest savanna zone between the rainforest and the desert. The lifeblood of transhumance in West Africa, it is home to many important wildlife species and responds, with the vegetation, in dynamic ways to the drought, seasonal flooding and pestilence for which it is famous.